The SAP Agent won't start when SSO is enabled explicitly in the SNC settings of the R3 connection object. Please note that the issue only occurs if the JCO v3.1.6 is used. The issue does not occur with JCO v3.1.2.
Error message in the agent's log:
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U02004999 Max. Number of RFC Connections: 'UNLIMITED'
U02004172 Error while calling function module 'RFC_SYSTEM_INFO'.
Initialization of repository destination <servername>102UC4_CPIC 63f446510592e7d452146123f51385c1d7b5a failed: Logon data incomplete.
U07001001 Charset used by the Agent: 'ISO-8859-15
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Release : 21.x
The issue is fixed from SAP Agent 21.0.5.